1. Field of the Invention
The present invention relates generally to measurement and monitoring of the coolant level in nuclear power reactors when the reactor is depressurized, such as during an outage for maintenance, inspection and refueling of the reactor.
2. Discussion of Background
Commercial, nuclear power reactors use water for three distinct purposes: for heat transfer, for neutron moderation to facilitate fission, and for radiation shielding. During reactor operation, the water, acting as a heat transfer medium, or "coolant," captures the energy generated by the fissioning of nuclear fuel as heat. In its role as a neutron moderator, the water molecules slow down the high energy neutrons released in a fission event so that they are more likely to cause a next fission event in a sequence of fission events, which sequence is the so-called chain reaction that enables the reactor to continue to produce power.
Water shields workers from radiation from "spent" or used nuclear fuel stored at reactors in "pools" and from radiation emanating from fuel in the core when the part of the core is being replaced during a refueling outage.
Reactors are generally pressurized during operation so that, at reactor temperatures, the coolant remains liquid, or at least partly liquid. Some reactors, called pressurized water reactors (PWRs), operate at pressures sufficiently high so that the coolant remains a liquid and does not turn to steam. Upon leaving the pressure vessel, the heated coolant is conducted to a heat exchanger where it transfers the heat it absorbed from the fuel to a second water system at lower pressure. The water in the secondary water system flashes to steam for use in driving stream turbines.
Other reactors, called boiling water reactors (BWRs), operate at somewhat lower pressures than PWRs so that two phase flow--water and steam--occurs directly in the reactor vessel. The steam is extracted from the pressure vessel and conducted from there to turbines. In both kinds of reactors, the coolant is circulated using coolant pumps and is kept at operating elevated pressure using pressurizers.
During a reactor refueling outage, the reactor pressure vessel is depressurized and the reactor head is removed for access to the core of nuclear fuel and vessel internal components and structures. Fuel in the core is replaced and maintenance and inspections of the reactor system can be performed during this time. Although the coolant level is lowered, the nuclear core in the pressure vessel is kept covered with coolant to remove "decay heat." Nuclear fuel, even after the chain reaction has been stopped, still generates and radiates considerable heat from the radioactive decay of the fission fragments locked inside the fuel elements. This decay heat must be removed by having a residual heat removal system circulate water through the core continuously. The coolant, in addition to being needed to absorb and remove this decay heat, also acts as a shield by attenuating radiation given off by the radioactive fuel, so that those engaged in refueling operations or otherwise near the core for inspection and maintenance are exposed to much less radiation than they would otherwise be were the core not covered with coolant, and, for that matter, covered to a depth that is high enough so that the residual heat removal system pump does not lose suction. Therefore, both for attenuating radiation and for decay heat removal, maintaining the level of coolant above the core during refueling is vital.
To maintain and monitor the coolant level in unpressurized power reactors, tubes of narrow diameter are typically used to conduct a small portion of the coolant to another location in the reactor containment, removed from the pressurizer and coolant loops. The coolant tubes are connected to the coolant loop piping at several locations and to the instrumentation connection located on the pressurizer. At this remote location, a portion of the coolant tubing is replaced with clear tubing so that the level of coolant in the tubes can be visually inspected. The coolant level in this "sight tube" corresponds to the actual level in the reactor system. At some reactors, a differential pressure device connected between two points in the coolant system produces an electrical signal related to the coolant level. The signal is proportional to the difference in the pressure at the "wet" and "dry" sides. In addition to sight tubes and differential pressure transducers, ultrasonic level sensors are used in the coolant loops when the coolant level falls below the top of the coolant loop pipe.
However, these systems are inherently inaccurate and can cause serious conditions to occur. Part of the inherent inaccuracy results from the differences in the relative size of the coolant piping compared to the instrument tubing. Also, if air intrudes into the "wet" side or water intrudes into the "dry" side of a differential pressure system, or if the dry side is exposed to a positive or negative pressure, such as when an air evacuation system is connected to the pressurizer or when the coolant level is increased, the levels indicated by these systems will be false.
Ultrasonic systems are also inaccurate, especially for higher draining rates where advance warning would be crucial, and they can confuse water droplets on the piping walls with the water level.
Furthermore, there is a lag in time from when the coolant level changes and the time when that change appears in a sight tube because of the low driving head between the level indication system and the coolant system. When the system is being drained, a lag in the change of the level means that the sight tube will give a false high signal; when the system is being refilled, it will give a false low signal. Both false signals are unconservative; that is, the error is potentially harmful and not simply an error that does no harm or one where the actual level of the coolant is safer than indicated. A false high signal can result in loss of residual heat cooling; a false low signal can result in spilling of coolant, which can result in contamination of equipment and personnel. Consequently, changes need to be made in small, slow increments so that the level detector has time to register the new coolant level before any further changes can be made. If too great a change is made, the realization of true level may not occur until shutdown cooling has been lost. Loss of shutdown core cooling has serious safety implications, yet there have been two hundred reported events since 1980 regarding failure of the maintenance drain down level indicating systems and a resulting loss of residual heat removal. These events have occurred because of discrepancies between the actual coolant level and the level shown by the level-indicating system.
Because of the consequence of errors in measuring liquid levels, the rate of level change is limited by utility operating procedures. This self-imposed requirement for slow level change carries with it a cost to the utility operating the nuclear power plant. Lowering and raising the liquid level are done at the beginning and at the end of the outage, both critical path events: their duration adds directly to the length of the outage. While the outage takes place, the utility may have to buy replacement power, typically at a cost of $500,000 to $750,000 per day, approximately $20,000 to $30,000 per hour.
Consequently, there remains a need for an accurate, rapid coolant level measuring and monitoring system for nuclear power reactors.